Ocean warming pattern shapes wildfire trends across two continents

Two of the world's most fire-prone regions have been moving in opposite directions for decades. Burned forest area in the United States Southwest has surged dramatically since the 1980s, while eastern Australia has not followed the same upward curve, despite both regions warming under climate change. A study published in Environmental Research Letters now offers a significant part of the explanation: a long-running shift in the tropical Pacific Ocean.

The research, led by Tess Wei-Ping Jacobson, a postdoctoral fellow at NASA's Goddard Institute of Space Studies who completed her doctorate at Columbia University's Lamont-Doherty Earth Observatory, identifies a multidecadal pattern in Pacific sea-surface temperatures. Since the 1980s, the western tropical Pacific has warmed considerably faster than the eastern Pacific. That asymmetry, the study finds, has quietly reshaped the fire risk on opposite sides of the ocean.

“"We found that what's happening in the tropical Pacific over longer timescales, not just during El Niño and La Niña, is another significant piece of the puzzle." — Tess Wei-Ping Jacobson, NASA Goddard Institute of Space Studies”

How ocean temperatures shape fire on land

The mechanism runs through atmospheric dryness. The study focused on a measure called vapour pressure deficit, which captures how strongly the atmosphere draws moisture from soil and plants. Higher deficit means drier conditions, and drier conditions mean forests burn more readily. Human-caused warming has increased vapour pressure deficit in both the US Southwest and eastern Australia, but the Pacific trend has pushed the two regions in opposite directions on top of that shared baseline. According to the study, the Pacific pattern added about 22% to the dryness-linked increase in burned area in the US Southwest, while reducing it by around 19% in eastern Australia.

The researchers analysed burned forest area and climate records across both regions from 1984 to 2022, combining direct observations with a large ensemble of climate model simulations. In the US Southwest, burned area rose by more than 3,000% in the interior and more than 1,000% in the coastal zone that includes much of California over that period. Phys.org reports that the Pacific trend moderated, but did not reverse, warming-driven increases in atmospheric dryness in eastern Australia, meaning the continent remains at serious long-term risk even if the ocean pattern has offered some relative relief in recent decades.

Implications for international fire cooperation

The practical stakes extend beyond either country's borders. Australia and the United States share firefighting personnel and equipment through an international partnership that works partly because their fire seasons have traditionally peaked at different times of year, Australia's in the southern hemisphere summer and the US's in the northern hemisphere summer. Columbia Climate School's reporting on the study notes that the overlap between those fire seasons has already begun to increase, which could strain the bilateral arrangement that both countries rely on when fires exceed local capacity.

“"A better understanding of those longer-term Pacific changes could help both countries plan for future wildfire risk." — Columbia Climate School summary of the study's findings”

The broader concern is about forecasting. El Niño and La Niña, the periodic warming and cooling of the central and eastern Pacific, are already closely monitored for their effects on rainfall and fire risk. A strong El Niño is currently developing, with NASA satellites confirming the 2026 event continued to strengthen in June. But the Jacobson study highlights a slower, multidecadal layer of Pacific variability that has received far less attention from fire managers, even though it appears to have meaningfully shifted baseline risk in two of the world's most economically significant fire landscapes. The authors stress that future wildfire risk will depend on both continued anthropogenic warming and the uncertain long-term trajectory of tropical Pacific sea-surface temperature patterns.

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